1. Field of Invention
This invention relates to thermally bonded nonwoven webs containing thermoplastic fibers or a mixture of thermoplastic and non-thermoplastic fibers that have been mechanically modified under specific process conditions so as to produce a finished web that is softer, less stiff, conformable and displays a significant and commercially valuable degree of elasticity. In one aspect the invention relates to nonwoven webs and laminates of nonwoven webs. In another aspect the invention relates to a method and apparatus for mechanically and thermally modifying thermally bonded nonwoven webs containing thermoplastic or a mixture of thermoplastic and non-thermoplastic fibers.
2. Status of Prior Art
Nonwoven textiles have created a large industry in response to the demand for inexpensive materials to replace woven textiles for use in disposable products in many fields. These include disposable sanitary protection products including adult and infant diapers, sanitary napkins, medical products such as masks, operating gowns, head covers, operating drapes, protective work-wear such as coveralls head covers and masks, and personal use items such as underwear.
Nonwovens, although inexpensive, have some negative aspects. They are not as strong nor as tough as traditional woven textiles. They tend to be much stiffer and less flexible than woven textiles with little or no elasticity, conformability or drapability.
The most popular nonwovens today are fabricated from webs of thermoplastic fibers. These webs may be made from mechanically laid fibers or fibers extruded directly from thermopolymers in the molten state. Regardless of the web formation method the fibrous webs thus produced have very little strength until the fibers composing the web are thermally bonded together. This is done by with heated embossed press rolls or other thermal means to join the fibers at their intersections which provides the required strength.
A major deficiency of nonwovens is their lack of elasticity or stretch, toughness, softness, and conformability. Toughness is an important factor in the durability and utility of disposable products. This is important from a utility aspect even though those products may be disposed of in a short period of time. Softness is also important especially in disposable diaper coverstock, disposable medical and industrial apparel, disposable sheets and pillow cases and any of the myriad uses of nonwovens where the nonwoven comes in contact with the skin.
The ability of a material to stretch and recover is a desirable quality in any fabric whether it be woven or nonwoven since it improves the toughness, conformability and fit of the resultant products. This property is generally called elasticity. In actual use a material only needs 30 to 50% recoverable stretch to provide adequate service in disposable apparel. As an example, disposable underwear with a 24 inch waist will only have to stretch 50% to fit over 36 inch hips.
Some of the attempts to provide stretch or elasticity and increased toughness and conformability have been to incorporate elastomerics into nonwovens. This is accomplished using films, bands, or threads of natural or synthetic rubber. Nonwoven disposable products using elastomerics are very expensive and consequently have found only limited use in the industry.
One method used to provide stretch or conformability was to crepe the fabric using various means of longitudinal mechanical compression. This provides some elongation in the direction of creping but no changes in tensile strength or toughness. A serious deficiency is that after elongation a creped material has very poor recovery characteristics. Creping also decreases the softness of the material.
U.S. Pat. No. 5,244,482 to Hassenboehler et al (1993) discloses a process which uses very high strain rates to laterally consolidate the precursor web with resultant reductions in average pore size and narrowing of the pore size distribution. Very high strain rates are required to change the morphology of the nonwoven and create the large changes in pore size. A degree of elasticity is created but the resultant fabric is stiff and the elastic modulus is low. Additionally, this patent places significant limitations on the precursor web""s physical properties as to crystallinity, thermoplastic fiber content, fiber diameter, random fiber deposition, isotropic tensile properties, and low tensile elongation to break.
The very high strain rates taught in U.S. Pat. No. 5,244,482 in order to provide high lateral consolidation of the fibers results in a web with gross changes in its morphology. These changes are manifested as reduced average pore size and an increased packing density which results in significant improvements in filtration efficiency.
U.S. Pat. No. 5,244,482 does not anticipate the use of very low strain rates and in fact places a lower limit on preferred strain rates that are 5 to 10 times higher than the present invention.
U.S. Pat. No. 5,244,482 is similar to U.S. Pat. No. 5,053,066 which also deals with post treatment of webs to change structure for filtration applications by the application of very high strain rates to a precursor thermoplastic web.
U.S. Pat. No. 4,048,364 to Harding et al (1977) discloses the use of high strain rates to increase the tensile strength of a ribbon of meltblown polypropylene fibers which in its pre-treated state must have a little or no fiber crystallinity or orientation. It is not noted whether elasticity is developed.
U.S. Pat. Nos. 5,441,550 and 5,443,606 to Hassenboehler et al (1995) disclose the same process as U.S. Pat. No. 5,244,482 but use different precursor webs.
The use of high strain rate drawing is well known and is practiced in the film industry to orient film to give increase strength and toughness. A typical stretching roll arrangement is shown in U.S. Pat. No. 4,408,974 to Comerio (1983).
Another well known example of high strain rate drawing is the drawing of thermoplastic textile fibers from the melt through a die using a series of Godet rolls wherein each successive set of pull rolls runs at consecutively higher speeds.
The web of the present invention is manufactured by elongating a nonwoven web under very low strain rates and carefully controlled thermal process conditions. This has the unexpected result of creating a high degree of elasticity within the precursor web and significant improvement in softness and conformability. This is accomplished without large changes in average pore size or pore size distribution which decrease softness and conformability.
Surprisingly, this result is not dependent on the properties of the precursor web as is the case with the prior art. The only criteria is that the precursor web be thermally bonded and contain at least 70% thermoplastic fibers with the remainder of the fibers being nonthermoplastic. The process works with meltblown, spunbond, and carded thermally bonded nonwovens as well as with laminates containing two or more of those aforementioned nonwovens and laminates of the aforementioned nonwovens and thermoplastic films.
The method of the present invention involves subjecting a thermally bonded nonwoven web containing at least 70% thermoplastic fibers to elongating forces at a carefully controlled low strain rate while the web is at a temperature of no more than 70 degrees F above its plastic point. The low strain rate elongation may be carried out in either the machine direction or the cross-machine direction using any of the precursor webs indicated above. The resultant web displays a surprisingly high and commercially valuable degree of elasticity. The resultant web is softer, less stiff and displays improved web toughness compared to the precursor web. The elasticity is developed in a direction perpendicular to the direction of elongation.
In both the machine direction and the cross machine direction cases a high degree of elasticity is developed using anisotropic precursor webs and a strain rate of less than 9.5 in./in./min. Both of these characteristics further distinguish this invention over U.S. Pat. No. 5,244,482.
Accordingly, besides the objects and advantages of the process and web described in our above patent, several objects and advantages of the present invention are:
(a) provide a process which produces soft, conformable elastic nonwoven materials which can overcome many of the objections to the use of nonwovens in disposable apparel and other products where the material comes into contact with the user""s skin.
(b) provide a process which produces soft, conformable and elastic nonwoven web materials at high rates of speed to make this post production process economical and commercially viable.
(c) provide a process that is very versatile in the choice of precursor webs.